The present invention relates to a sensor assembly and method used for analysis of nitrogen dioxide. The sensor assembly includes a light emitting diode as radiation source, a sample chamber containing the gas to be measured, and at least one radiation detector located to receive the radiation emitted by the radiation source and passed through the gas to be measured. The invention also relates to the analysis of gases containing nitrogen dioxide and/or nitric oxide.
The analysis of small concentrations of nitrogen dioxide (NO2) is typically measured using a sensor for nitric oxide (NO) based on chemiluminescence. The procedure is first to measure the background concentration of NO and then to convert all NO2 to NO in an oven and after that remeasure the NO content. The difference between the two readings gives an estimation of the concentration of nitrogen dioxide. The method is sensitive to below ppm levels but the measurement cannot be performed reliably in real-time especially if the NO background level changes. This is the case when measuring NO delivered to a patient or when measuring the endogenic NO concentration produced in a patient.
Nitrogen dioxide is a highly toxic gas often produced from NO in the presence of oxygen. As a precaution, it has therefore been proposed to monitor inhaled NO2 concentrations to prevent damage to the patient. It is advisable to measure the NO2 concentration on a breath-to-breath basis, meaning that a response time of about 200 ms is required. This is difficult to meet using a NO2 to NO converter.
The same applies for the commonly used electrochemical sensor. It is small and relatively cheap but the response time is too long and the sensitivity low. Such a cell also has a limited lifetime and other gases may interfere with the desired gas measurement. Infrared absorption could also be used to measure NO2 but the sensitivity is low unless the measuring chamber is very long. A long chamber means increased volume and increased response time. Therefore, this method cannot be used clinically. A good review of all mentioned measuring methods is found in S. C. Body et al.: Nitric oxide: Delivery, Measurement, and Clinical Application (Journal of Cardiothoracic and Vascular Anaesthesia, Vol. 9, No. 6, 1995: pages 748-763).
It is well known that nitrogen dioxide is one of the few gases that absorbs light in the visible region, see e.g. the reference T. C. Hall, Jr. and F. E. Blacet: Separation of the Absorption Spectra of NO2 and N2O4 in the Range of 2400-5000 A (The Journal of Chemical Physics, Vol. 20, No. 11, 1952: pages 1745-1749). To the eye the gas looks brownish in low concentrations. As an aside, it may be noted that this accounts, at least in part, for the brownish color of smog. The gas can even become almost black in high concentrations at elevated temperatures. At room temperature (21xc2x0 C.) the gas is a mixture of the monomeric NO2 and the dimeric N2O4 in equilibrium. About 16% is in form of NO2. At 100xc2x0 C. this fraction has increased to 90% and at about 120xc2x0 C. practically all molecules are in the monomeric state. Only the monomeric NO2 absorbs visible light above 400 nm so gas temperature is an important parameter unless the isobestic point at about 350 nm, where both types have similar absorption, can be used. The absorption band is broad and almost continuous between about 300 nm and 600 nm with a region of high absorption without disturbance from N2O4 approximately between 390 nm and 450 nm. The absorption can be measured using either a mercury source with emission lines either at 405 nm or 436 nm or a tungsten lamp filtered to give a wavelength band at the blue end of the spectrum. The problem with these measuring systems is that the source is slow which means that it is not possible to utilize the benefits of a system with high frequency modulation. Therefore, these systems, in addition to being quite bulky, power consuming, and complicated, are not suitable for fast measurements of very low concentrations ( less than 1 ppm) of nitrogen dioxide. In addition, at least the mercury source has limited lifetime.
In U.S. Pat. No. 4,857,735, a spectrophotometer incorporating at least one light emitting diode is presented for conventional measurement of solutions. The absorption is measured through a short cuvette and the reference signal is obtained from measurement of a blank solution. This means that the measurement, in practice, is slow. A high light intensity is essential for measuring solutions with sometimes very high absorbances. Therefore high current pulsing with a small duty cycle is important.
However, for gas measurements there is no need for such intensive pulses because the absorption is always small. The high current would produce excessive current noise which would badly disturb the gas measurement and raise the minimum level of detectable gas concentration. Because of the slow measurement, no means for correction of fast drifts are provided and because of the nature of the measurement no means for compensating changes in source intensity or detector sensitivity are present. The instrument as such would consequently not be suitable for measuring gases such as nitrogen dioxide.
An object of the present invention is to provide a sensor assembly and method for the analysis of nitrogen dioxide with a radiation source efficiently emitting light in the blue end of the visible spectrum. A second object of the invention is to provide a sensor assembly, the source of which is modulatable at a frequency that is high compared to changes measurable e.g. in breathing gas. A third object of the invention is the provision of such a sensor assembly, which is both cheap and simple to construct and long-lasting and thus reliable for monitoring of toxic levels of nitrogen dioxide. A fourth object of the invention is to provide such a sensor assembly, which can measure small concentrations of nitrogen dioxide without interference from other gases, especially nitric oxide. A fifth object of the invention is the provision of such a sensor assembly, which in combination with a sensor for nitric oxide, can measure small concentrations of both nitrogen dioxide and nitric oxide in real time.
A salient feature of the sensor assembly according to the invention is that it can be made small, simple, fast, long-lasting, and reliable. The reason for this is that a light radiating diode is used as light source in the sensor assembly. Blue emitting light emitting diodes, commonly known as LEDs, with high enough optical power and long enough lifetime have not been possible to produce commercially until recently. The emission spectrum of such a light emitting diode fits well to the absorption spectrum of nitrogen dioxide which means that all the benefits of light emitting diodes can be utilized. Since the only gas the sensor assembly reacts to is nitrogen dioxide and since the sensor assembly does not change the composition of the measured gas it is possible to made fast measurements of small concentrations of both nitrogen dioxide and nitric oxide by combining the sensor assembly with a sensitive and fast sensor for nitric oxide, preferably a sensor based on chemiluminescence.
Various other features, objects, and advantages of the invention will be apparent from the following detailed description and the drawings.